The present invention relates to an ion source, and more particularly to an ion source capable of generating both a positive ion beam and a negative ion beam.
An ion source for generating an ion beam, which is used for forming a semiconductor body and other substrates, and can be used in a secondary ion mass spectrometer, an ion scattering spectrometer and others.
Conventionally, a liquid metal ion source such as described in a Japanese patent application specification (examined publication No. Sho 58-33649) and a surface ionization type ion source have been known. The operation principle of the liquid metal ion source is as follows. An ion source material for generating a desired ion species is supplied to the tip of a needle-like ion emitter chip in the molten state. Then, a strong electric field is formed between the ion emitter chip and an ion extracting electrode, and thus a sharp cone is made of the molten ion source material at the tip of the emitter chip so that the surface tension of the molten ion source material balances with the stress due to the electric field. In this state, an ion beam is emitted from the tip of the cone by the field evaporation mechanism.
The liquid metal ion source has the following drawbacks. (1) It is difficult to take out an ion from a sublimate material. (2) The initial energy of emitted ions is as high as 5 to 50 eV. (3) It is difficult to take out negative ion species from this type of ion source. (4) It is difficult to obtain an ion beam from a component element of a gas. (5) The angular distribution of ion emission is small, and it is basically impossible to obtain a high-intensity ion beam.
While, in a surface ionization type ion source, an ion emitter is made of one of mono-atomic metals having a large work function such as W and Ir, only ions of elements having a low ionization potential such as Cs and Li can be emitted from the ion emitter. That is, the surface ionization type ion surface has a drawback that only the ion of an electropositive element can be emitted. Regarding the general knowledge of the ion source, U.S. Pat. application Ser. No. 578,206 is hereby incorporated by reference.